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Cattle living at altitudes greater than 1,500 m are susceptible to the development of high altitude disease (HAD) and a higher risk of death. Pulmonary arterial pressure (PAP) is used as an indicator of an animal's susceptibility to HAD. Measurements of PAP on cattle at lower elevations are generally considered less reliable indicators of HAD when compared to measurements taken at higher elevations, yet little evidence has been published evaluating this relationship. Therefore, the objective of this study was to evaluate PAP measurements from high and low elevations using a multivariate approach. We hypothesized that PAP measurements from lower elevations have a less than perfect relationship to PAP measurements from high elevations. Data collected from 2009 to 2017 was obtained from a multi-breed seedstock database that included PAP measurements and associated information (i.e., PAP testing date and elevation). The average PAP measurement was 44.08 ± 10.75 mmHg, with minimum and maximum observations of 32 and 149 mmHg, respectively. The age when PAP measurements were taken ranged from 9 to 21 months with an average age of 15.9 ± 2.8 months. Elevation ranged from 1,555 m to 2,407 m with a bimodal distribution. Therefore, animals with PAP measurements from elevations above 2,000 m were classified as high elevation (HPAP; n = 1,369) and animals with PAP measurements below 2,000 m were considered low elevation (LPAP; n = 1,243). A two-trait animal model was used to estimate genetic parameters and EBV for both LPAP and HPAP traits. The model contained the fixed effects of degree of outcross, breed percent, and PAP age as covariates, and sex and contemporary group (PAP date and yearling management combined) categories. Breed effects were included as covariates of breed percentages for Angus, Charolais, South Devon, Gelbvieh, Simmental, and “Other” breeds. Animal was included as a random effect and a 3-generation pedigree consisting of 11,573 animals was used. Heritability for LPAP and HPAP were 0.26 ± 0.08 and 0.37 ± 0.10, respectively, with a genetic correlation of 0.79 ± 0.23. Sire EBV for HPAP were regressed on corresponding sire EBV for LPAP, resulting in a regression coefficient of 0.705 ± 0.002 (P< 0.0001). These results suggest that a strong relationship exists between LPAP and HPAP, but the relationship is not perfect. The high genetic correlation between the traits suggest that measurements taken at lower elevations can still serve as an indicator of PAP measurements collected at higher elevations.

Pulmonary hypertension can develop due to hypoxia-induced pulmonary vascular remodeling leading to right ventricular hypertrophy and edema, a condition known as brisket or high altitude disease (HAD). Pulmonary arterial pressure (PAP) is used at high altitude regions ( > 1500 m) as an indicator of an animal's susceptibility to pulmonary hypertension and HAD. Cattle located at altitudes greater than 1500 m with PAP measurements between 31 and 43 mm Hg are considered to have a low risk of developing HAD at most elevations. To date, there has been no reported research for the effects of heterosis on PAP and therefore, the objective of this study were to examine the effect of heterosis on PAP measurements. Classically, heterosis is most beneficial for survival and fertility related traits; therefore, we hypothesized that increased heterozygosity would decrease an animal's PAP phenotype. Data collected from 2009 to 2015, was obtained from a multibreed seedstock database with an average PAP measurement of 44.56 ± 11.58, and a minimum and maximum of 32 and 149 mm Hg. Data included PAP records (n= 2001), PAP testing date, yearling management code, date of birth, sex and breed. A mixed animal model was used to estimate the effect of heterosis on PAP with the model containing degree of outcross and PAP age as covariates; and contemporary group (i.e., combination of PAP date and yearling management code) and sex as categorical fixed effects. Animal was included as a random effect. A 3-generational pedigree consisting of 9353 animals was used to estimate genetic parameters for PAP (h2= 0.29 ± 0.07). Breed effects were included as covariates of breed percentages for Angus, Red Angus, Charolais, South Devon, Gelbvieh, Simmental and “Other” breeds. The effect of breed on PAP had a range of 8.75 mm Hg. The estimated regression coefficient for PAP on heterosis was −0.02 ± 1.31 mm Hg/percent outcross (P < 0.155). These results indicate that heterozygosity has no effect on PAP measurements, although other multibreed populations should be examined. As a result we would reject our hypothesis that an increase in heterosis would decrease PAP.